Bactericidal silicon dioxide doped with silver

ABSTRACT

Pyrogenically prepared silica doped with silver or silver oxide is prepared by feeding an aerosol into a flame such as is used for the preparation of pyrogenic silica, mixing the aerosol homogeneously with gas mixture before the reaction, then allowing the aerosol/gas mixture to react in a flame. The resulting pyrogenic silicas doped with silver or silver oxide are separated from the gas stream. The pyrogenic silica doped with silver or silver oxide by means of an aerosol can be used as a bactericidal filler.

CROSS-REFERENCE TO RELATED APPLICATION

This is a divisional of U.S. application Ser. No. 09/658,505, filed onSep. 8, 2000, which is now U.S. Pat. No. 6,423,331 (issued on Jul. 23,2002), which claims priority to German Application No. DE 199 43 057.8,filed on Sep. 9, 1999, which disclosure is incorporated herein byreference.

FIELD OF THE INVENTION

The invention relates to a pyrogenically prepared silicon dioxide dopedwith silver or silver oxide by means of an aerosol and havingbactericidal properties, and to a process for its preparation and itsuse. The invention relates also to use of this doped silicon dioxide indispersions or as a filler in rubber and silicone rubber.

BACKGROUND OF THE INVENTION

It is known to dope pyrogenically prepared silica in one step in a flameusing a special process. (DE 96 50 500 A1). That process is acombination of high-temperature flame hydrolysis with pyrolysis. Thatdoping process is to be distinguished from the older, so-called“co-fumed process”, in which the gaseous starting materials (for exampleSiCl₄ gas and AlCl₃ gas) are pre-mixed and burnt together in a flamereactor.

SUMMARY OF THE INVENTION

In the doping process, an aerosol is fed into a flame in which apyrogenic oxide is produced by flame hydrolysis, the aerosol containinga salt of the compound to be doped.

It has now been found that if silver salts dissolved in water are usedas the starting material for the aerosol, a doped pyrogenic silicahaving bactericidal properties is obtained as the product.

The invention provides a pyrogenically prepared silica doped with silveror silver oxide by means of an aerosol. The silica is characterized inthat the base component is a silica prepared pyrogenically by flameoxidation or, preferably, by flame hydrolysis. The silica is doped witha doping component of from 0.0001 wt. % up to 20 wt. %, the dopingamount preferably being in the range from 1 to 10,000 ppm, and thedoping component is a salt or a salt mixture of a silver compound ormetallic silver or mixtures thereof. The BET surface area of the dopedoxide is from 1 to 600 m²/g, preferably in the range from 40 to 400m²/g.

The invention further provides a process for the preparation of thepyrogenically prepared silica doped with silver or silver oxide by meansof an aerosol, which process is characterized in that an aerosol is fedinto a flame such as is used for the pyrogenic preparation of silica byflame oxidation or, preferably, by flame hydrolysis, the aerosol ismixed homogeneously with the gas mixture used for the flame oxidation orthe flame hydrolysis before the reaction, then the aerosol/gas mixtureis allowed to react in the flame and the resulting pyrogenicallyprepared silicas doped with silver or silver oxide are separated fromthe gas stream in a known manner. An aqueous solution containing saltsor salt mixtures of silver or the metal itself in dissolved or suspendedform or mixtures thereof is used for the preparation of the aerosol. Theaerosol is produced by atomization by means of a two-component nozzle orby a different method of aerosol production.

The following may be used as salts: Ag(NO₃), Ag₂(SO₄), Ag₂O, or thesalts may be complexed with complexing agents or ammonia.

The process of flame hydrolysis for the preparation of pyrogenic oxidesis known from Ullmanns Enzyklopädie der techn. Chemie 4th edition, Vol.21, p. 464.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of the doping apparatus.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The doped silicon dioxide according to the invention and the process forits preparation and use are described below with reference to FIG. 1 andthe following Example.

The core element of the apparatus is a burner of known type for thepreparation of pyrogenic oxides.

The burner 1, shown in FIG. 1 includes a central pipe 2 which opens intoa nozzle 3 from which the main gas stream flows into the burner chamber10,in which chamber the gas burns. This inner nozzle 3 is surrounded bya ring nozzle or jacket 4, from which secondary hydrogen flows.

In the central pipe 2 there is a further axial pipe 5, which endsseveral centimeters before the outlet of the nozzle 3 of the centralpipe 2. The aerosol is fed into the axial pipe 5.

The aerosol, which is an aqueous silver salt solution, is produced in anaerosol generator 6, which may be an ultrasonic atomizer.

The silver salt/water aerosol produced in the aerosol generator ispassed by means of a light carrier gas stream through a heating zone 7,in which the entrained water vaporizes, leaving small salt crystals infinely divided form in the gas phase.

Secondary air is fed into the burner chamber through inlet 8.

EXAMPLE 1 Preparation of a Pyrogenic Silica Doped with Silver or SilverOxide

4.44 kg/h of SiCl₄ are vaporized at about 130° C. and transferred to thecentral pipe of a burner of known type. 3 Nm³/h of primary hydrogen and7.5 Nm³/h of air are additionally fed into the central pipe.

The gas mixture flows out of the inner nozzle of the burner and burns inthe burner chamber and the water-cooled flame tube adjacent thereto.

0.5 Nm³/h of secondary hydrogen are fed into the ring nozzle.

12 Nm³/h of secondary air are additionally fed into the burner chamber.

A second gas stream flows out of the axial pipe 5 into the central pipe2.

The second gas stream consists of an aerosol which is produced in aseparate atomizing unit by ultrasonic atomization. The aerosol generatoratomizes 725 g/h of 5% aqueous silver sulfate solution. The silversulfate aerosol is guided through a heated line with the aid of thecarrier gas of 0.5 Nm³/h of air, the aqueous aerosol being converted attemperatures of about 180° C. into a gas and a salt crystal aerosol.

The reaction gases and the resulting pyrogenic silica doped with silveror silver oxide are drawn through a cooling system by the application ofreduced pressure, and the particle gas stream thereby cools to a rangeof about 100° to 160° C. The solid is separated from the waste gasstream in a filter or cyclone (not shown).

The pyrogenically prepared silica doped with silver or silver oxide isobtained in the form of a finely divided white powder. In a furtherstep, any hydrochloric acid residue still adhering to the silica isremoved therefrom at elevated temperature by treatment with aircontaining water vapor.

The BET surface area of the pyrogenic silica doped with silver is 206m²/g.

The preparation conditions are summarized in Table 1, further analyticaldata of the silica so obtained are given in Table 2.

TABLE 1 Experimental conditions for the preparation of pyrogenic silicadoped with silver or silver oxide Pri- mary Sec. H₂ Aero- air air coreH₂ sol Air SiCl₄ Nm³/ Nm³/ Nm³/ jacket Salt amount aerosol BET kg/h h hh Nm³/h solution kg/h Nm³/h m²g 4.44 7.5 12 3 0.5 5% 0.725 0.5 206aqueous Ag₂SO4 Explanation: Primary air = amount of air in the centralpipe; Sec. air = secondary air; H₂ core = hydrogen in the central pipe;Aerosol amount = mass flow rate of the salt solution converted intoaerosol form; Air aerosol = amount of carrier gas (air) for the aerosol;

TABLE 2 Analytical data Tamped Ag₂O BET pH value density content m²/g 4%sus. g/l wt. % Example 206 4.13 24 1.72 Explanation: pH 4% sus. = pHvalue of the 4% aqueous suspensionAdvantages of the silica according to the invention:The silica according to the invention doped with silver or silver oxidehas bactericidal properties.

The pyrogenic silica doped with silver or silver oxide by means of anaerosol can be used as a bactericidal filler especially in theproduction of paints or coatings, in the paper industry, as acatalytically active substance, as a starting material for thepreparation of dispersions, as a polishing material (CMP applications),as a ceramics base material, in the electronics industry, as a fillerfor polymers, as a starting material for the production of glass orglass coatings or glass fibers, as a release auxiliary even at hightemperatures, in the cosmetics industry, as an absorber material, as anadditive in the silicone and rubber industry, for adjusting the rheologyof liquid systems, for heat protection stabilization, as a heatinsulating material, as a flow auxiliary, as a filler in the dentalindustry, as an adjuvant in the pharmaceuticals industry or in the paintindustry.

1. A process for the preparation of pyrogenic silica doped with silveror silver oxide, comprising: vaporizing silicon tetrachloride; mixingvaporized silicon tetrachloride with hydrogen and air; preparing anaerosol using as a starting material a salt solution containing salts orsalt mixtures of silver or silver metal in dissolved or suspended formor mixtures thereof, the aerosol being produced by atomization by meansof a two-component nozzle; mixing the aerosol homogeneously with a gasmixture used for flame oxidation or flame hydrolysis and the vaporizedsilicon tetrachloride with hydrogen and air; feeding the mixture of theaerosol and gas and the vaporized silicon tetrachloride with hydrogenand air into a flame used for preparation of pyrogenic oxides by flameoxidation or flame hydrolysis; reacting the mixture of aerosol and gasand the vaporized silicon tetrachloride with hydrogen and air in theflame; and separating pyrogenic silica doped with silver or silver oxidefrom a resulting gas stream.
 2. A process according to claim 1, whereinthe aerosol is produced in an aerosol generator.
 3. A process accordingto claim 2, wherein the aerosol generator comprises an ultrasonicatomizer.
 4. The process according to claim 1, wherein the mixture ofaerosol and gas react to form a pyrogenic oxide doped with 0.0001 wt. %to 20 wt. % silver or silver oxide.
 5. The process according to claim 1,wherein the separated pyrogenic oxides doped with silver and silveroxide have a BET surface area from 1 to 600 m²/g.